1. Field of the Invention
The present invention relates to a thin film deposition apparatus, and more particularly to a thin film deposition apparatus for depositing thin films on substrates by means of material gas introduced into a processing chamber and plasma generated therein, in order to produce semiconductor devices.
2. Description of the Related Art
An example of a conventional thin film deposition apparatus will be explained by referring to FIG. 5. One type of such a thin film deposition apparatus is a plasma CVD (chemical vapor deposition) apparatus for depositing a thin silicon oxide film identical to or nearly identical to a thin thermal oxide film with respect to film quality, in which oxygen gas (O.sub.2) is used for generating plasma and monosilane gas (SiH.sub.4) is used as a material gas for forming the thin film.
As shown in FIG. 5, the plasma CVD apparatus includes: a bell jar 11 used as a vessel in which plasma is generated; a power-supply unit 12 for supplying electric power into the bell jar 11; a deposition chamber 13 related to the bell jar 11 spatially to make a common space; a magnetic field generating unit 14 for generating multi-cusped magnetic fields in the deposition chamber 13, which is placed around the deposition chamber 13; a vacuum pumping unit 15 for evacuating both the bell jar 11 and the deposition chamber 13; a first gas introduction unit 16 for introducing the oxygen gas (O.sub.2) into the deposition chamber 13; and, a second gas introduction unit 17 for introducing the monosilane gas (SiH.sub.4) into the deposition chamber 13.
The bell jar 11 of the plasma CVD apparatus is made of a dielectric material and is used as a plasma generating chamber. In practice, the bell jar 11 is made of quartz glass which has a pipe-like shape with a closed upper end and an open lower end, and whose inside diameter is about, e.g., 100 mm. The open lower end of the bell jar 11 is connected to an upper wall of the deposition chamber 13. The power-supply unit 12 shown in FIG. 5 comprises a high frequency power source 21, a matching box 22 and loop antennas 23 arranged around the bell jar 11. The high frequency power source 21 outputs high frequency power of 13.56 MHz, for example. The power-supply unit 12 is not, however, limited to the above-mentioned high frequency power unit, and other types of power-supply units may be used. The deposition chamber 13 is formed by using an aluminum alloy member of a cylindrical shape with a bottom, whose length in an axial direction is 230 mm and whose inside diameter is 360 mm. The magnetic field generating unit 14 placed around the deposition chamber 13 consists of twelve permanent magnet rods, each of which has two magnetic poles and is parallel with the axis of the deposition chamber 13. The vacuum pumping unit 15 comprises an evacuation chamber 31, two valves 32a and 32b arranged in serial stages and evacuation pumps 33. The evacuation pumps 33 include a turbo molecular pump 33a as a main evacuation pump and a dry pump 33b as a backing pump.
A substrate holder 42 used for holding a substrate 41 is disposed in a lower portion of the deposition chamber 13. The substrate holder 42 has a structure 43 for circulating heat exchange media therein and a temperature detector (not shown in the figures), both of which are used in controlling the temperature of the substrate holder 42 as desired. In addition, the substrate holder 42 is connected to a high frequency power source 44 which can supply a bias power to the substrate 41 on the substrate holder 42. The high frequency power source 44 supplies a high frequency power of 400 KHz, for example.
FIG. 6 shows a detailed structure of the second gas introduction unit 17 and a part of the structure is shown in cross section. A gas supply end-portion of the second gas introduction unit 17, which is disposed in the inside of the deposition chamber 13, is a ring-shaped pipe member 51 that supplies the material gas into the deposition chamber 13 through a number of gas outlets 52 formed therein. The gas outlets 52 are formed in the inside of the pipe member 51 so as to be arranged at regular intervals, for example. The pipe member 51 has, for example, at least one gas carrying pipe 53 attached to its outside as shown in the drawing. The ring-shaped pipe member 51 is fixed in the deposition chamber 13 by a supporting member 54 for supporting the gas carrying pipe 53 and several other supporting members 55 having the similar structures. The supporting member 54 additionally has a tube structure to connect both sides of the deposition chamber 13, and is fixed to a wall of the deposition chamber 13. The supporting member 55 is similarly fixed to the wall of the deposition chamber 13. The monosilane gas introduced through the second gas introduction unit 17 is supplied to the pipe member 51 through the supporting members 54 and the gas carrying pipe 53, and is injected into the interior space of the deposition chamber 13 through the gas outlets 52 in the pipe member 51 so that the momosilane gas is supplied to the surface of the substrate 41. The monosilane gas ejected from the gas outlets 52 flows in the direction of an arrow 56 shown FIG. 6.
When a process of depositing the thin silicon oxide film onto the surface of the substrate is continued in the above-mentioned conventional plasma CVD apparatus, the process results in such a phenomenon that the thin silicon oxide film is also deposited on the interior surface 13a of the deposition chamber 13 since it is in contact with the plasma generated therein, and further, the thin silicon oxide film will be deposited on the whole surfaces of the ring-shaped pipe member 51, the supporting members 54 and 55, and the gas carrying pipe 53, because the plasma moves to contact all surfaces in the deposition chamber 13. If the thin silicon oxide films are deposited on the various interior parts in the deposition chamber 13, the deposited thin film will generate a lot of undesirable particles before long because the thin films on the parts have strong interior stresses and therefore particles are undesirably peeled off. These particles cause surface defects on the thin silicon oxide film deposited on the substrate 41. The surface defects cause deterioration of value of the substrate with the defective thin silicon oxide film.
To explain in more detail, since the deposition chamber 13 has a relatively larger cylindrical shape with the upper and bottom ends as mentioned above, in which the height (length in the axial direction of the deposition chamber 13) is 230 mm and the inside diameter is 360 mm, the interior surface 13a in the side wall of the deposition chamber 13 can be shaped in the cylindrical shape without any sharp angle portions or small curvature portions. In consequence, the thin silicon oxide films deposited on the interior surface 13a have a low internal stress and therefore can be deposited to become relatively thick before starting to peel off. On the other hand, the pipe member 51, the supporting members 54 and 55, and the gas carrying pipe 53 are formed in shapes having small curvature portions, for example, a pipe shape whose radius of curvature is about 5 mm. Further connecting portions between the pipe member 51 and the supporting member 54 include sharp angle portions, or the supporting members 54 and 55 are formed by metal plates having sharp angle portions. Accordingly, when the thin silicon oxide film is deposited onto the surfaces of the portions having small radius of curvature or sharp angle portions, the internal stress of the film becomes very strong. Therefore, the problem of peeling-off in a very small thickness occurs when using the conventional deposition chamber 13.